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Subnet delegation and API Management

When updating some network related ARM templates I stumbled on the concept of subnet delegation.

What is subnet delegation?

Subnet delegation enables you to designate a specific subnet for an Azure PaaS service of your choice that needs to be injected into your virtual network. When you delegate a subnet to an Azure service, you allow that service to establish some basic network configuration rules for that subnet, which help the Azure service operate their instances in a stable manner. One of the advantages is that the service can configure for example routing policies or network security groups. 

Subnet delegation and API Management

When I was updating our virtual network/subnet templates I stumbled on the delegation section.

"delegations": [
  {
    "id": "string",
    "properties": {
      "serviceName": "string"
    },
    "name": "string"
  }
]

After doing some research about the concept I opened the subnet configuration in the Azure portal I noticed that I can select API Management from the pulldown. In the online documentation I cannot find any information about the benefits for selecting this option. 

If you integrate an API Management service in an internal network and you route the internet traffic through an Azure Firewall you will have to make quit what adjustments in the terms of route tables and NSGs. These are documented in the common network configuration issues section of the page which describes How to use Azure API Management with virtual networks. So using subnet delegation whould be nice if it solves this common issues.

Microsoft support

Because I could not find any information in the Microsoft documentation and I had not the time to do R&D with this setting I opened a support call. The conclusion of the communication with the support engineer is as follows:

  • The subnet delegation setting is doing anything currently (November 2020).
  • The idea is that when Microsoft have the implementation backing it you would delegate the subnet to the service and Microsoft will enforce the policies as defined in the Common Network Configuration Issues so the API Management service will stay operating.


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